We examined the time course of plasma lipoprotein and apolipoprotein concentration changes in six volunteers upon changing from the standing to sitting or supine positions and back. Recumbence produced an 8-10% decrease in TC,HDL-C, apolipoproteins A-I and B, and Lp(a), and a 17% decrease in triglycerides. The changes were half as great upon sitting. The changes were reversed upon standing, and all changes were complete within 20-40 minutes.